Current sense amplifiers, memory devices and methods

ABSTRACT

A current sense amplifier may include one or more clamping circuits coupled between differential output nodes of the amplifier. The clamping circuits may be enabled during at least a portion of the time that the sense amplifier is sensing the state of a memory cell coupled to a differential input of the sense amplifier. The clamping circuits may be disabled during the time that the sense amplifier is sensing the state of a memory cell at different times in a staggered manner. The clamping circuits may be effecting in making the current sense amplifier less sensitive to noise signals.

TECHNICAL FIELD

Embodiments of this invention relate to memory devices and methods, and,more particularly, current sense amplifiers and methods and memorydevices and methods using current sense amplifiers.

BACKGROUND OF THE INVENTION

Memory devices typically include a plurality of memory cells, which maybe arranged in an array of intersecting rows and columns. The locationof each cell in the array may be indicated with an address, such as arow address and/or a column address. The memory cells may be read byselecting a row of memory cells with a corresponding row address. A bitof stored data may then be coupled from all or some of the columns inthe addressed row.

Memory devices may store data using a variety of techniques. Forexample, dynamic random access memory (“DRAM”) devices may use memorycells that are essentially small capacitors that store data based on theamount of charge stored in the capacitors. When a memory cell is read,the memory cell may be coupled to a digit line so that the charge of thecapacitor is shared with a charge of the digit line to which it iscoupled. This charge sharing results in a small change in the voltage towhich the digit line had been previously charged. A respective senseamplifier coupled to each digit line may be utilized to sense a smalldifferential between the voltage on the digit line and the voltage on acomplementary digit line. Voltage sense amplifiers have long been usedin this manner to sense voltage differentials between complementarydigit lines. However, voltage sense amplifiers may not function wellover a wide range of power supply voltages, particularly at the low endof the range. For example, at low supply voltages the sense amplifiermay not respond quickly enough to the voltage differential being sensed.

Another type of sense amplifier that may function better than voltagesense amplifiers in some cases, particularly at low supply voltages, arecurrent sense amplifiers. Current sense amplifiers sense a differentialin the current flow from complementary digit lines resulting fromcoupling one of the digit lines to a memory cell, such as a DRAM memorycell. However, at lower supply voltage, even current sense amplifiersmay be unable to respond to a current differential with sufficientspeed, and they may tend to incorrectly respond to noise signals,thereby producing invalid data results. This problem may be particularlyacute as the electrical characteristics of electrical components used incurrent sense amplifiers may charge responsive to process, supplyvoltage and temperature variations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of a prior art currentsense amplifier.

FIG. 2 is a schematic diagram of a current sense amplifier according toone embodiment.

FIG. 3 is a timing diagram showing one embodiment for operating thecurrent sense amplifier of FIG. 2.

FIG. 4 is a schematic diagram of a current sense amplifier that is thecomplement of the current sense amplifier of FIG. 2.

FIG. 5 is a schematic diagram of a current sense amplifier according toanother embodiment.

FIG. 6 is a schematic diagram of a current sense amplifier that is thecomplement of the current sense amplifier of FIG. 5.

FIG. 7 is a schematic diagram of a current sense amplifier according tostill another embodiment.

FIG. 8 is a schematic diagram of a current sense amplifier that is thecomplement of the current sense amplifier of FIG. 7.

FIG. 9 is a block diagram of an embodiment of a memory system that mayuse the current sense amplifiers of FIGS. 2 and 4-8 or a current senseamplifier according to some other embodiment.

DETAILED DESCRIPTION

An embodiment of a conventional current sense amplifier 10 is shown inFIG. 1. The current sense amplifier 10 includes a pair of NMOS currentsensing transistors 14, 16, which have their gates and drainscross-coupled to each other. Respective sources of the transistors 14,16 may be coupled to the drains of respective NMOS transistors 20, 22,which may receive an active high Enable signal at their gates. Theinterconnected drains of the transistors 14, 20 may receive an inputsignal In at a first input node 26, while the interconnected drains ofthe transistors 16, 22 may receive a complementary input signal InB at asecond input node 28. The sources of the transistors 20, 22 may becoupled to a first supply voltage, such as ground. The drains of thetransistors 14, 16 may be coupled to the drains of respective PMOStransistors 30, 32, which may receive an active low EnableB signal attheir respective gates. The interconnected drains of the transistors 14,30 may provide an output signal OutB at a first output node 36, whilethe interconnected drains of the transistors 16, 32 may provide acomplementary output signal Out at a second output node 38. The sourcesof the transistors 30, 28 may be coupled to a second supply voltage,such as V_(DD). Finally, a clamping circuit 40 may be coupled betweenthe drains of the sensing transistors 14, 16, and the gate of thetransistor 42 receives an equilibration signal EQ.

In operation, the input nodes 26, 28 may be coupled to respectivecomplementary digit lines (not shown). The sense amplifier may beenergized by applying a high ENABLE signal to the gates of thetransistors 20, 22, and applying a low ENABLEB signal to the gates ofthe transistors 30, 32. However, before a current differential is sensedby the sense amplifier 10, the clamping circuit 40 is turned ON byapplying a high EQ signal to its gate, thereby coupling the drains ofthe sensing transistors 14, 16 to each other so that they are atsubstantially the same voltage when sensing of a memory cell commences.A memory cell may then be coupled to one of the input nodes 26, 28. Forexample, if a memory cell storing a logic “1” is coupled to the inputnode 28, a current i may be supplied to the input node 28, while nosimilar current is supplied to the input node 26. However, since thetransistors 20, 22 are biased with the same voltage and the voltages ofthe digit lines coupled to the input nodes 26, 28 are substantially thesame, the currents through the transistors 20, 22 are substantiallyequal. Thus, if a current of I flows through each of the transistors 20,22, the current flowing through the transistor 14 will also be I, butthe current flowing through the transistor 16 will be I−i. Therelatively little current flowing through the transistor 16 compared tothe current flowing through the transistor 14 may cause the voltage atthe output node 38 to be less than the voltage at the output node 36,Further, gain provided by the sensing transistors 14, 16 may result inpositive feedback, which amplifies this differential voltage. Morespecifically, the lower voltage at the output node 38 applied to thegate of the transistor 14 may cause the transistor 14 to be turned ON toa lesser degree than the transistor 16, thereby further raising thevoltage at the output node 36. This increased voltage at the output node36 is applied to the gate of the transistor 16 to turn ON the transistor16 to a greater degree than the transistor 14, thereby further reducingthe voltage at the output node 38.

One disadvantage of the current sensing amplifier 10 is shown in FIG. 1is that it may be susceptible to being responsive to noise signals,thereby resulting in invalid data. This sensitivity to noise signals mayresult from the drains of the sensing transistor 14, 16 beingequilibrated to the same voltage prior to sensing. Therefore, if a noisesignal is received at one of the input nodes 26, 28 the change in thevoltage at one input node compared to the other may cause the sensetransistors 14, 16 to respond because of the high gain of thetransistors 14, 16. This problem might be reduced by reducing the gainof the transistors 14, 16, but doing so might reduce the speed at whichthe sense amplifier 10 can sense a differential current.

An embodiment of a current sense amplifier 100 that may avoid some orall of these problems is shown in FIG. 2. The current sense amplifier100 may use many of the same components that are used in the prior artcurrent sense amplifier 10 shown in FIG. 1, and these common componentsmay function in essentially the same manner. Therefore components in thecurrent sense amplifier 100 of FIG. 2 have been provided with the samereference numerals as the corresponding components in the current senseamplifier 10 of FIG. 1, and an explanation of their operation will notbe repeated. The current sense amplifier 100 of FIG. 2 differs from thecurrent sense amplifier 10 of FIG. 1 by the use of two clamping circuits110, 112 rather than a single clamping circuit 40 used in the senseamplifier 10, and in the use of a pass gate having both a PMOStransistor 116 and an NMOS transistor 118 instead of the single NMOStransistor 42 used in the clamping circuit 40 of the sense amplifier 10.The NMOS transistor 118 in the clamping circuit 110 may be coupled toone output of a control circuit 120, and the PMOS transistor 116 in theclamping circuit 110 may coupled to the same output of a control circuit120 through an inverter 124. In the same manner, the NMOS transistor 118in the clamping circuit 112 may be coupled to another output of thecontrol circuit 120, and the PMOS transistor 116 in the clamping circuit112 may be coupled to the same output of a control circuit 120 throughan inverter 128. The use of both the PMOS transistor 116 and the NMOStransistor 118 in each clamping circuit 110, 112 may allow the outputnodes 36, 38 to be coupled to each other regardless of the voltages ofthe output nodes 36, 38. In contrast, the single NMOS transistor 42 usedin the clamping circuit 40 may require that the output nodes 36, 38 beat a voltage that is lower than the voltage used for the EQ signal by atleast the threshold voltage of the transistor 42.

The current sense amplifier 100 of FIG. 2 also differs from the currentsense amplifier 10 of FIG. 1 in the manner in which the clampingcircuits 110, 112 are operated compared to the manner in which theclamping circuit 40 is operated. Instead of using the prior arttechnique of actuating the clamping circuit 40 only before a memory cellis to be sensed, the control circuit 120 may actuate the clampingcircuit 110, 112 during the time that the memory cell is sensed. Morespecifically, as shown in FIG. 3, in one embodiment the sense amplifier100 is enabled at time t₀ by transitioning the Enable signal high andtransitioning the EnableB signal low. At that time, the clampingcircuits 110, 112 are in an actuated condition by the respective highClamp1 and Clamp2 signals. A short time later at time t₁, a memory cellis coupled to one of the input nodes 26, 28, and the differentialcurrent In/InB at the input nodes 26, 28 starts to increase nagatively.The differential current In/InB and reaches a peak at time t₄, andthereafter begins to decrease. Just after time t₁, the differentialcurrent In/InB is relatively small so a prior art sense amplifier 100might be susceptible to responding to noise signals, thereby providinginvalid data. However, the sense amplifier 100 may be relativelyinsensitive to noise signals because the actuated clamping circuits 110,112 may limit the gain of the sense amplifier 100 at that time.Thereafter, when the differential current In/InB has increasedsignificantly, the control circuit deactivates the clamping circuit 110by transitioning the Clamp1 signal low, thereby increasing the gain ofthe sense amplifier 100. The gain of the sense amplifier 100 may againbe increased at a later time t₂ without a significant risk of respondingto noise signals because of the increased magnitude of the differentialcurrent In/InB and/or an increase in the differential output signalOut/OutB after the sense amplifier has responded to the differentialcurrent In/InB. Furthermore, after the magnitude of the differentialcurrent In/InB and/or the differential output signal Out/OutB hasfurther increased, the gain of the sense amplifier 100 may be furtherincreased without a significant risk of responding to noise signals.Therefore, the clamping circuit 112 may be deactivated at time t₅ bytransitioning the Clamp2 signal low. Finally, the sense amplifier 100may be disabled at time t₆ by transitioning the Enable signal low andthe EnableB signal high. Thus, by staggering the deactivation of theclamping circuits 110, 112, the gain of the sense amplifier 100 may begradually increased as the sense amplifier becomes gradually lesssusceptible to responding to noise signals.

Although the embodiment of the sense amplifier 100 shown in FIG. 2 usestwo clamping circuits 110, 112, it should be understood that otherconfigurations may also be used. For example, the sense amplifier 100may use a single clamping circuit operated in a manner similar to theoperation explained with reference to FIG. 3 in which the clampingcircuit is active for at least a portion of the time that a memory cellis being sensed. By way of further example, the number of clampingcircuits used in a current sense amplifier may be greater than two, andthe times at which all or some of them may be disabled may be staggered.

An embodiment of a current sense amplifier 130 that is complementary tothe current sense amplifier 100 is shown in FIG. 4. Therefore thecomponents of the current sense amplifier 130 have been provided withreference numerals that differ from the corresponding components in thecurrent sense amplifier 100 only by the suffix “a” in the eventcomplementary components are used. In all other respects the structureand operation of the current sense amplifier 130 may be the same as thestructure and operation of the current sense amplifier 100.

Another embodiment of a current sense amplifier 140 is shown in FIG. 5,and a complementary current sense amplifier 150 is shown in FIG. 6. Thecurrent sense amplifiers 140, 150 may differ from the current senseamplifiers 100, 130, respectively, by separating the clamping circuits110, 112 from each other with resistors 144, 146. The resistors 144, 146may be used to adjust the output resistance and hence the gain of thesense amplifiers 140, 150 when the clamping circuit 112, 112 a isenabled.

Another embodiment of a current sense amplifier 160 and its complement170 are shown in FIGS. 7 and 8, respectively. The current senseamplifiers 160, 170 differ from the current sense amplifiers 140, 150,respectively, by omitting the clamping circuit 110, 110 a so that only asingle clamping circuit 112, 112 a is used. As explained above, a singleclamping circuit 112, 112 a may be used instead of multiple clampingcircuits as long as the single clamping circuit is enabled at leastduring a portion of the time that a memory cell is being sensed, such asduring the portion of the time that a memory cell is initially beingsensed.

FIG. 9 is block diagram of an embodiment of a memory system 200 that mayuse the current sense amplifiers of FIGS. 2 and 4-8 or a current senseamplifier according to some other embodiment. The memory system 200 mayinclude an array 202 of memory cells, which may be, for example, DRAMmemory cells, SRAM memory cells, flash memory cells, or some other typesof memory cells. The memory system 200 may include a command decoder 206that receives memory commands through a command bus 208 and generatescorresponding control signals within the memory system 200 to carry outvarious memory operations. Row and column address signals may be appliedto the memory system 200 through an address bus 220 and provided to anaddress latch 210. The address latch 210 may then output a separatecolumn address and a separate row address to the memory array 202. Thecolumn address decoder 228 may select digit lines extending through thearray 202 corresponding to respective column addresses. The row addressdecoder 222 may activate respective rows of memory cells in the array202 corresponding to received row addresses. The selected digit linecorresponding to a received column address may be coupled to aread/write circuit 230. The read/write circuit 230 may include currentsense amplifiers 234 to provide read data to a data output buffer 234via an input-output data bus 240. The current sense amplifiers may beany of the current sense amplifiers 100, 130, 140, 150, 160, 170 shownin FIGS. 2 and 4-8, respectively, or a current sense amplifier accordingto some other embodiment. Write data may be applied to the memory array202 through a data input buffer 244 and the memory array read/writecircuitry 230. The command decoder 206 may respond to memory commandsapplied to the command bus 208 to perform various operations in thememory array 202. In particular, the command decoder 206 may be used togenerate internal control signals to operate the control circuit 120used in each of the current sense amplifiers 100, 130, 140, 150, 160,170.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from theinvention. Such modifications are well within the skill of thoseordinarily skilled in the art. Accordingly, the invention is not limitedexcept as by the appended claims.

1. A current sense amplifier, comprising: first and second current sense transistors; an output node coupled to a drain of one of the first and second current sense transistors; an input node coupled to a source of one of the first and second current sense transistors; a clamping circuit coupled between respective drains of the first and second current sense transistors, the clamping circuit being configured to couple the respective drains of the first and second current sense transistors to each other responsive to a clamp signal; and a control circuit coupled the clamping circuit, the control circuit being configured to apply the clamp signal to the clamping circuit during at least some of the time that the current sense amplifier is sensing current coupled to or from the input node of the current sense amplifier.
 2. The current sense amplifier of claim 1 wherein the control circuit is configured to apply the clamp signal to the clamping circuit during an initial period when the current sense amplifier initially senses current coupled to or from the input node of the current sense amplifier.
 3. The current sense amplifier of claim 1 wherein the control circuit is configured to terminate applying the clamp signal to the clamping circuit before the current sense amplifier has completed sensing current coupled to or from the input node of the current sense amplifier.
 4. The current sense amplifier of claim 1 wherein the current sense transistors comprises respective NMOS transistors.
 5. The current sense amplifier of claim 1 wherein the clamping circuit comprises a first clamping circuit, and the clamp signal comprises a first clamp signal, and wherein the current sense amplifier further comprises a second clamping circuit coupled between respective drains of the first and second current sense transistors, the second clamping circuit being configured to couple the respective drains of the first and second current sense transistors to each other responsive to a second clamp signal, the control circuit being configured to generate both the first clamp signal and the second clamp signal during at least some of the time that the current sense amplifier is sensing current coupled to or from the input node of the current sense amplifier.
 6. The current sense amplifier of claim 5 wherein the control circuit is configured to apply the first and second clamp signals to the first and second clamping circuits, respectively, during an initial period when the current sense amplifier initially senses current coupled to or from the input node of the current sense amplifier.
 7. The current sense amplifier of claim 6 wherein the control circuit is configured to terminate applying the first and second clamp signal to the clamping circuit at different times before the current sense amplifier has completed sensing current coupled to or from the input node of the current sense amplifier.
 8. The current sense amplifier of claim 5, further comprising a resistor coupled between the first and second clamping circuits, the resistor being coupled between the second clamping circuit and the drain of one of the first and second current sense transistors.
 9. The current sense amplifier of claim 1 wherein the clamping circuit comprises a pass gate.
 10. The current sense amplifier of claim 9 wherein the pass gate comprises: a NMOS transistor coupled between respective drains of the first and second current sense transistors; a PMOS transistor coupled between respective drains of the first and second current sense transistors; and an inverter coupled between a control terminal and a gate of one of the NMOS transistor and the PMOS transistor, the control terminal being coupled to the other of the NMOS transistor and the PMOS transistor.
 11. The current sense amplifier of claim 1, further comprising: a first enable transistor coupled between the first current sense transistor and a supply voltage; and a second enable transistor coupled between the second current sense transistor and the supply voltage.
 12. A memory device, comprising: an array of memory cells; a read data path coupled to a set of data bus terminals; and a plurality of current sense amplifiers coupled to respective sets of memory cells in the array, each of the plurality of current sense amplifiers being configured to couple read data to one of the data bus terminals in the set responsive to sensing current from a respective one of the memory cells, each of the current sense amplifiers comprising: first and second current sense transistors, a drain of one of the first and second current sense transistors being coupled to the read data path, and a source of one of the first and second current sense transistors being coupled to the array; a clamping circuit coupled between respective drains of the first and second current sense transistors, the clamping circuit being configured to couple the respective drains of the first and second current sense transistors to each other responsive to a clamp signal; and a control circuit coupled the clamping circuit, the control circuit being configured to apply the clamp signal to the clamping circuit during at least some of the time that the current sense amplifier is sensing current coupled to the current sense amplifier from one of the memory cells in the array or from the current sense amplifier to one of the memory cells in the array.
 13. The memory device of claim 12 wherein the control circuit is configured to apply the clamp signal to the clamping circuit during an initial period when the current sense amplifier initially senses current coupled to the current sense amplifier from one of the memory cells in the array or from the current sense amplifier to one of the memory cells in the array.
 14. The current memory device of claim 13 wherein the control circuit is configured to terminate applying the clamp signal to the clamping circuit before the current sense amplifier has completed sensing current coupled to the current sense amplifier from one of the memory cells in the array or from the current sense amplifier to one of the memory cells in the array.
 15. The memory device of claim 12 wherein the clamping circuit comprises a first clamping circuit, and the clamp signal comprises a first clamp signal, and wherein the current sense amplifier further comprises a second clamping circuit coupled between respective drains of the first and second current sense transistors, the second clamping circuit being configured to couple the respective drains of the first and second current sense transistors to each other responsive to a second clamp signal, the control circuit being configured to generate both the first clamp signal and the second clamp signal during at least some of the time that the current sense amplifier is sensing current coupled to the current sense amplifier from one of the memory cells in the array or from the current sense amplifier to one of the memory cells in the array.
 16. The memory device of claim 15 wherein the control circuit is configured to apply the first and second clamp signals to the first and second clamping circuits, respectively, during an initial period when the current sense amplifier initially senses current coupled to the current sense amplifier from one of the memory cells in the array or from the current sense amplifier to one of the memory cells in the array.
 17. The memory device of claim 16 wherein the control circuit is configured to terminate applying the first and second clamp signal to the clamping circuit at different times before the current sense amplifier has completed sensing current coupled to the current sense amplifier from one of the memory cells in the array or from the current sense amplifier to one of the memory cells in the array.
 18. The memory device of claim 15, further comprising a resistor coupled between the first and second clamping circuits, the resistor being coupled between the second clamping circuit and the drain of one of the first and second current sense transistors.
 19. The memory device of claim 12 wherein the array of memory cells comprises an array of dynamic random access memory cells.
 20. The memory device of claim 12, further comprising: a first enable transistor coupled between the first current sense transistor and a memory device supply voltage; and a second enable transistor coupled between the second current sense transistor and the memory device supply voltage.
 21. A method of operating a current sense amplifier having first and second current sense transistors each of which includes a source and a drain, the method comprising coupling respective drains of the first and second current sense transistors to each other during at least some of the time that the current sense amplifier is sensing current coupled to or from a source of one of the first and second current sense transistors.
 22. The method of claim 21 wherein the act of coupling the respective drains of the first and second current sense transistors to each other comprises coupling the respective drains of the first and second current sense transistors to each other during an initial period when the current sense amplifier is initially sensing current coupled to or from a source of one of the first and second current sense transistors.
 23. The method of claim 21 wherein the act of coupling the respective drains of the first and second current sense transistors to each other comprises terminating the coupling of the respective drains of the first and second current sense transistors to each other before the current sense amplifier has completed sensing current coupled to or from a source of one of the first and second current sense transistors.
 24. The method of claim 21 wherein the act of coupling the respective drains of the first and second current sense transistors to each other comprises coupling the respective drains of the first and second current sense transistors to each other through first and second coupling paths that are different from each other.
 25. The method of claim 21 wherein the act of coupling the respective drains of the first and second current sense transistors to each other comprises: coupling the respective drains of the first and second current sense transistors to each other through first and second coupling paths that are different from each other during at least some of the time that the current sense amplifier is sensing current coupled to or from a source of one of the first and second current sense transistors; terminating the coupling of the respective drains of the first and second current sense transistors to each other through the first coupling path at a first time before the current sense amplifier has completed sensing current coupled to or from a source of one of the first and second current sense transistors; and terminating the coupling of the respective drains of the first and second current sense transistors to each other through the first coupling path at a second time before the current sense amplifier has completed sensing current coupled to or from a source of one of the first and second current sense transistors, the second time being different from the first time.
 26. The method of claim 25 wherein the first and second path have respective impedance between the respective drains of the first and second current sense transistors that are different from each other. 